Theretofore, a number of techniques have been proposed for the method for forming insulating films on the surfaces of electromagnetic steel plates or the compositions for forming insulating films (which may hereinafter be referred to as the insulating film compositions). In the main part of these techniques, use is made of the insulating film (which may hereinafter be simply called the film) compositions containing as the principal component a chromate and a phosphate or a combination thereof to form inorganic films. The film properties have been improved by the following various procedures, viz., the addition of metal oxides such as MgO, ZnO and CaO which react with free acids present in the film compositions to enhance the insulating properties of the films, the addition of small amounts of glycerin and saccharides for promoting the aforesaid reactions and forming oxides insoluble in water and organic solvents, and further the addition of boric acid, water-glass, condensed phosphates, silica and the like. (Aqueous solutions of inorganic film-forming substances containing a chromate or a phosphate as mentioned above may hereinafter be called the inorganic film compositions.) However, the inorganic films occupied by and composed of for the most part such inorganics have been disadvantageous in that they excel in insulating properties, but are yet inferior in punching properties, adhesion at the time of processing and the like.
In recent years, weight has been given to the punching properties of electromagnetic steel plates. To improve the punching properties, there have been attempted the method for forming organic films on the above inorganic films, the method for forming mixed inorganic/organic films by the application of the film compositions containing admixtures of inorganic film-forming elements such as chromates or phosphates (which may hereinafter be referred to as the inorganic components) and organic film-forming resin (which may hereinafter be called the organic component or simply the resin), and the like.
However, the former method involves two coating steps, and unavoidably gives rise to an increase in the cost.
The latter method is promising, however in order to carry out this use should be made of a mixed liquid prepared by mixing the inorganic film composition containing chromates or phosphates as the inorganic components with an aqueous emulsion in which the organic film-forming resin is emulsified and dispersed in water. This has offered the following problems.
First, a problem is a short pot life that the emulsified and dispersed state of the resin become unstable under the influence of the chromate showing a strong oxidative effect and, in several hours after mixing, the mixed liquid gels due to the resin aggregation, and in consequence, the mixed liquid often becomes unable to be used during coating works.
Second, a problem is that the film properties needed for the insulating films on electromagnetic steel plates such as insulating, corrosion resistance and punching properties become incomplete, since a number of film defects such as crateriform pinholes or foams are caused in the films formed by the application and drying of the aforesaid mixed liquid on electromagnetic steel plates.